There is a need to screen semiconductor single crystal wafers (e.g., CdS, HgCdTe, GaAs) quickly and accurately in order to process them into detectors at the high rates required to meet projected production schedules at the lowest possible cost. For example, the present method of screening wafers requires making electrical contact to the wafer by physically bonding a dissimilar metal to the wafer surface. This technique is labor intensive and frequently causes mechanical and electrical problems that result in inaccurate and non-repeatable results. In contrast, the system described in this disclosure requires no physical contacts, is not labor intensive and can map the wafer with high spatial resolution thereby identifying small areas of nonuniformity that could not otherwise be found by conventional screening methods. Subject invention can characterize the wafer quickly with a much higher spatial resolution than conventional methods, thus greatly reducing time and cost now incurred from processing areas of a wafer unsuitable for detectors.